Method for preparing solid-state polymer zinc-air battery

ABSTRACT

This invention relates to a method for fabricating solid-state alkaline polymer Zn-air battery, which consists of a zinc-gel anode, an air cathode electrode, and alkaline polymer electrolyte. The formulation of said zinc gel anode is similar to that of alkaline Zn—MnO 2  battery. The zinc gel anode contains a mixture of electrolytic dendritic zinc powders, KOH electrolyte, gelling agent and small amount of additives. The air cathode electrode is made by carbon gas diffusion electrode, which comprises two layers, namely gas diffusion layer and active layer. The active layer on the electrolyte side uses a high surface area carbon for oxygen reduction reaction and potassium permanganate and MnO 2  as catalysts for oxygen reduction. The diffusion layer on the air side has high PTFE content to prevent KOH electrolyte from weeping or climbing. Due to adequate amount of fresh air and oxygen supply, the air cathode electrode can run continuously. Theoretically, the polymer zinc-air battery is an accumulator if the cell has sufficient zinc powder and electrolyte, and the air cathode plays the role of energy transfer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for preparing solid-state polymerZn-air battery which uses environmentally friendly carbon material andzinc powder.

2. Description of the Related Art

Energy drives economic growth. It is also an important indicator gaugingthe strength and civilization of a country and living standard of itspeople. History illustrates that each innovative breakthrough in energytechnology brought significant and profound influence on productivityand advancement of the civilization, demonstrating the importance ofenergy technology and its major influence on emerging industries.

Environmental protection has become an issue the human society is highlyconcerned about in the 21st century. It is the core issue in mapping outstrategy for sustainable development and a key factor influencing theenergy policy and technological orientation of countries. At the sametime, it is a great propelling force behind the development of energytechnology. The gigantic energy system we built up in the 20th centurycan not meet the. requirements for high-efficiency, clean, economicaland safe energy system for the future. In short, energy development isfacing tremendous challenges ahead.

Energy production and consumption as well as global climate change areclosely related to the greenhouse effect on earth. The current energysystems contribute to at least half of the greenhouse effect, that is,from carbon dioxide released after the burning of fossil fuel, whichprovides four fifths of the world's energy. The consumption of fossilfuel is continuously on the rise at the rate of 3% a year. Therefore thedischarge of carbon dioxide also increases at the same speed. It isestimated that discharge of carbon dioxide will increase two folds by2002 and three folds by 2025. Thus elevating energy utilization rate anddeveloping alternative energy sources are highly important subjects inthe 21st century.

Up to now, the majority of energy conversion is achieved throughthermo-mechanical process. But constrained by Carnot cycle,thermo-mechanical process not only results in low conversion rate,leading to waste of energy, but also generates large amount of dust,carbon dioxide, NO_(x), SO_(x) and other harmful substances as well asnoise, leading to the pollution of air, water and soil and seriouslythreatening the living environment of mankind.

SUMMARY OF THE INVENTION

To address the problem discussed above and in light that electrochemicalprocess is the most effective means of converting chemical energy intoelectric energy, this invention purports to provide a solid-statepolymer Zn-air battery that uses environmentally friendly materials.

Another objective of the invention herein is using solid-state polymerelectrolyte in place of conventional liquid KOH electrolyte andseparators to solve the battery leakage problem and allow the battery tobe applied in light, thin, short and small 3C products.

A further objective of the invention herein is the use ofelectrolytically-prepared dendritic zinc powders with large surface areain zinc electrode that offers greater power, greater discharge rate andhigher utilization percent of zinc; the solid-state polymer Zn-airbattery of this invention shows impressively high energy density byvolume and by weight at various testing conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the state of solid PVA-GF polymer electrolyte of thisinvention observed by Scanning Electron Microscope (SEM);

FIG. 2 is a flow chart for preparing PVA-GF polymer electrolyte of thisinvention;

FIG. 3 shows the dendritic structure of electroplated zinc powder atcurrent density of 250 mA/cm² and examined by SEM at 200×;

FIG. 4 shows the dendritic structure of electroplated zinc powder atcurrent density of 250 mA/cm² and examined by SEM at 500×;

FIG. 5 shows the dendritic structure of electroplated zinc powder atcurrent density of 250 mA/cm² and examined by SEM at 3000×;

FIG. 6 is the XRD of anode zinc powder in regular Zn-air battery;

FIG. 7 is the XRD of anode dendritic zinc powder of this invention;

FIG. 8 is a flow chart for preparing the dendritic zinc powder of thisinvention;

FIG. 9 is a flow chart for preparing the gelled zinc anode of thisinvention;

FIG. 10 is a sketch of air cathode electrode of this invention;

FIG. 11 is a flow chart for preparing the diffusion layer of airelectrode of this invention;

FIG. 12 is the simplified process flow for preparing air electrode(active layer included) of this invention;

FIG. 13 is a sketch of electrochemical testing of air electrode of thisinvention;

FIG. 14 is a structural diagram of solid-state polymer Zn-air battery ofthis invention,

FIG. 15 is an AC resistance-impedance graph of PVA-GF film of thisinvention under different temperatures environment;

FIG. 16 is the Arrhenius plot of alkaline PVA-GF polymer electrolyte ofthis invention;

FIG. 17 is the cyclic voltammetry diagram of alkaline PVA-GF polymerelectrolyte of this invention at different temperatures environment;

FIG. 18 shows the effect of storage time on the conductivity ofsolid-state PVA-GF polymer electrolyte of this invention;

FIG. 19 shows the potential versus time graph of electrolytic zincprepared under constant current density;

FIG. 20 shows the polarization curve of air electrode of this inventionat different temperatures environment;

FIG. 21 shows the AC resistance graph of air electrode of this inventionat different temperatures environment;

FIG. 22 is the discharge curves of different Zn-air batteries (usingdifferent separators);

FIG. 23 is the microscopic structure of PP/PE separator used by regularZn-air battery examined by SEM;

FIG. 24 is the microscopic structure of PVA-GF electrolyte of thisinvention examined by SEM;

FIG. 25 is the discharge curves of Zn-air battery of this inventionunder different discharge rates;

FIG. 26 is the discharge curves of Zn-air battery of this inventionunder different temperatures environment;

FIG. 27 is the AC resistance diagrams of Zn-air battery of thisinvention under different temperatures environment;

DETAILED DESCRIPTION OF THE INVENTION

Preparation of Solid-State Alkaline Polymer Electrolyte

Along with the development of new technology, different kinds of polymerbattery are now available in the market and applied in 3C products,turning thinner, lighter, and smaller products into market mainstream inthe near future. Battery using solid polymer electrolyte offers manyadvantages in terms of safety, workability, and use in high temperature.There is no need to worry about leakage of electrolyte due to improperpackaging or decrease of electrolyte in separator after the battery hasbeen idle for a while, and the battery will maintain good performanceunder high temperature. That is why solid-state polymer batteryrepresents a significant breakthrough in the future development ofbattery applications.

Polyvinyl alcohol (PVA) polymer is a water-soluble compound.

Glass fiber cloth (GF) is a fusion of silicon dioxide (SiO₂). Glassfiber yarn has flexibility and tensile strength increased by a dozenfolds in comparison with regular glass. When used for reinforcement,this material is usually in superfine fibrous state that offers strengthand excellent flexibility, and does not produce residual stressregardless of the shape of resulting product.

Glass fiber as reinforcement material possess the following properties:

1) High tensile strength which is twice that of steel wire having thesame mass.

2) Dimensional stability: Under maximum stress, its unit dimensionschanges by 3˜4% only.

3) High thermal resistance: It retains 50% of tensile strength under thetemperature of 343° C.

4) Superior corrosion resistance: It exhibits excellent corrosionresistance and brittleness property when in contact with the majority ofchemicals.

5) Excellent fire proofing: It does not burn (generate heat), norsmolder (generate smoke).

PVA polymer electrolyte has extremely high ion conductivity afterprocessing, but its mechanical strength is not as good as ordinary PP/PEseparators due to structural toughness. Thus glass fiber cloth is addedin the preparation of PVA polymer electrolyte to greatly improve itsmechanical strength up to five times that of ordinary separators andthermal stability without sacrificing its conductivity. It also solvesthe contraction problem after long-term storage. Due to the highmechanical strength of glass fiber cloth reinforced PVA polymer film, itis less prone to deformation during processing, charging and dischargingof battery or battery packaging. Under scan electron microscopeexamination, the surface of PVA polymer film is free of large pin holes,but has many small holes with 0.1-0.2 μm in size. As shown in FIG. 1,when used in zinc-air battery, it blocks the entry of zinc ion into theair cathode electrode when the zinc anode discharges, thereby preventingthe occurrence of short circuit problem that shortens the service lifeof battery. In addition, with the KOH electrolyte in gel state whendipped in PVA polymer, it helps solve leakage and corrosion problem ofbattery brought about electrolyte seeped through separator. Moreover,this polymer electrolyte retains high conductivity and electrochemicalstability.

The PVA-GF polymer electrolyte of this invention is prepared by addingpotassium hydroxide (KOH), water and glass fiber cloth to PVA solutionunder certain co-polymerization preparation conditions. Under ambienttemperature, the conductivity of this PVA-GF polymer electrolyte reaches10⁻¹ S/cm, indicating that zinc-air battery that uses this alkalinesolid polymer electrolyte will perform better than commercial zinc-airbatteries that use PP/PE separator. In addition, this polymerelectrolyte may come in different thickness, size and shape toaccommodate the battery requirements for size, capacity, and voltage.

The preparation of alkaline polymer electrolyte of this inventionconsists of five steps:

1) Select PVA and KOH materials and have PVA and KOH react with waterseparately;

2) Add the KOH solution to the PVA solution depending on the dissolutionof PVA in water under controlled temperature and time;

3) Terminate the reaction depending on the set reaction time and thedissolution status of the mixture and then coat the polymer of differentamounts on carrier or fiber glass to obtain films of desired thickness;

4) Control the film formation time, temperature and humidity to keepproper water content in the polymer film; and

5) Test the electrochemical property of solid alkaline polymer filmproduced thereof.

The procedure and method for preparing the PVA-GF polymer electrolyte ofthis invention are described in details as follows:

(1) Selection and Pre-Treatment of Raw Materials

Use PVA of 80˜99% purity with average molecular weight in the range of2,000˜120,000, and preferably between 5,000 and 10,000, in eithergranule or powder form. Use potassium hydroxide of 85% purity in eithergranule or powder form.

(2) Reaction Sequence

The ratio of reactants and reaction sequence will directly affect thecomposition of polymer film and film formation. If the weight percentageof PVA is too high, dissolution will become difficult and conductivitywill drop; if the weight percentage of PVA is too low, film formationmight not occur. If the weight percentage of potassium hydroxide is toohigh, the resulting poor structure will make film formation difficult.If both of these materials are fed at the same time, neither willdissolve. Thus the proportion and dissolution sequence of the reactantsare vital in the polymer film process. This inventor finds that mixing10˜20 wt % PVA with 50˜60 wt % water under ambient temperature and in aclosed environment for approximately two hours will result in completedissolution. At the same time, adding 15˜25 wt % potassium hydroxide to10˜20 wt % water under ambient temperature and in a closed environmentto undergo mixture and dissolution.

(3) Control of Polymer Blending Conditions

The temperature and time of polymerization reaction will affect thewater content of polymer film; the higher the water content, the higherthe conductivity. But polymerization will only occur under specifictemperature. Thus the control of polymerization time and reducing theloss of water are vital. This invention mixes completely the dissolvedPVA solution and the potassium hydroxide solution under ambienttemperature. At this time, white solid matter results. Mix it with thesolutions thoroughly and heat the solutions in closed container under50˜100° C. with the option of adding some micro or nano-particle oxides,such as γ-Al₂O₃, TiO₂, ZrO₂, and SiO₂ to improve the physical andchemical properties of the polymer. Let the reaction go on for about 30minutes until the solid matter is completely dissolved. Cool thesolution in atmosphere. After the solution is cooled, coat the alkalinepolymer fluid on the carrier (e.g. glass fiber cloth or PTFE membrane)to obtain film of desired thickness.

(4) Film Formation Conditions

Cut glass fiber cloth of proper size and lay it flat on the carriertray. Pour the viscous alkaline polymer solution into it and spread thesolution according to the desired film thickness. Put the carrier trayinto the temperature/ humidity chamber under 40˜80° C. and 30˜50 RH %(optimum conditions are 50˜60° C, and 20˜30 RH %) for about 30˜60 minuntil solid polymer film is formed. Then take out the carrier tray andleave it in atmosphere for 30 minutes before removing the film.

(5) Testing the Electrochemical Properties of Alkaline PolymerElectrolyte

(i) Testing of Conductivity

Measure the resistance of solid alkaline polymer electrolyte withAutolab FRA AC impedance analyzer and dipolar stainless steel electrodeswith frequency scan between 100 kHz˜0.1 Hz with amplitude of 10 mV. Alsocalculate the conductivity of the polymer electrolyte withσ = L/(R_(b) × A).At the left side high-frequency area of Nyquist graph, the impedancevalue that intersects Z′ axis with Z″ axis (capacitance) at zero is theresistance of polymer electrolyte film (R_(b)).

(ii) Testing of Electrochemical Stability

Use Autolab GPES system to measure the cyclic voltammetry of thispolymer electrolyte and other types of separators. The potential rangeis −1.5˜1.5V, the scan rate is 1 mV/s and stainless steel (SS-316) isused as working electrode and Hg/HgO electrode as a reference electrode.

(iii) Testing of Electrical Property of Battery

Assemble a Zn-air battery using the PVA-GF polymer electrolyte of thisinvention and a zinc electrode (−) and the air electrode (+); theelectrode area is about 6 cm² (2 cm×3 cm). Discharge current at 50 mA,100 mA and 200 mA, respectively and compare the electrical performanceof batteries with different separators.

(iv) Computation of Chemical Composition of PVA-GF Alkaline Polymer Film

Use mass balance method to compute the composition ratio of PVA-GFpolymer electrolyte before and after reaction.

(v) Computation of Activation Energy (E_(a))

Graph log a versus 1/T in Arrhenius plot to obtain slope, and thencalculate activation energy (E_(a)).σ=σ_(o) exp(−E _(a) /RT), or log σ=log σ_(o) −E_(a)/(2.303×1000R)×1/T  (1)

The preparation process flow for solid alkaline PVA-GF polymerelectrolyte is shown in FIG. 2.

Preparation of Zinc Gel Anode

Zn-air battery may replace the commercially available alkaline cells asa primary cell with high energy density. Zinc powder plays the mostimportant role in zinc-air battery, which decides capacity, currentdensity, flat discharge voltage, self-discharge rate and battery cost,essentially all factors that determine the performance of a battery. Toenhance the utilization rate of zinc powder, this invention aims todevelop dendritic zinc powder, for it offers good ductility, highersurface area, and smaller particles. After discharge of zinc electrode,the zinc powder converts into zinc oxide, zinc oxide powder may berecycled to form zinc powers. On the other hand, recycling can also helpreduce environmental pollution. Thus this invention uses alkaline KOHsolution as a solvent to dissolve zinc oxide and the resulting mixtureis electroplated under certain conditions to produce high porous.dendritic zinc powder. There are many control variables in thepreparation process, such as current density, concentration of zincoxide in the electrolyte, temperature, additives, mass transferconditions, and drying time. Thus the whole process must be carried outunder specific operating conditions to obtain dendritic zinc powder withoptimum electrical and chemical characteristics.

To make good performance of Zn-air battery, zinc powder is the keymaterial in deciding its service life and performance. Zinc powder usedby commercially available battery has large-size particles around300-600 μm and is widely distributed that keeps the battery from workingunder large current load and results in lower utilization percent ofzinc powder. This invention provides the method of preparing porousdendritic zinc powder with high specific surface area and low density,which may be applied in many alkaline battery, such as zinc-air batteryand zinc-system battery (e.g. Zn—MnO₂, Ni—Zn. Fe—Zn, Zn—Br₂, etc.), andmany recycled zinc oxide, thus improring the performance of battery,lowering costs, and friendly to the environment.

Process for Preparing Electrolytic Dendritic Zinc Powder

(1) Testing the Solubility of Zinc Oxide

Dissolve zinc oxide of different percentages into KOH solution of 1˜10Munder the temperature between 25˜60° C. and 50˜80 RH %, then measure thesolubility of zinc oxide. The solubility of ZnO in KOH is constrained bythermodynamic equilibrium. Experiments find that the solubility of ZnOin KOH solution is about 6-7%. Thus this invention uses 7 wt % of ZnO inpreparing dendritic zinc powder by electroplating.

(2) Solubility of Zinc Oxide

Remove the oxidized zinc anode from zinc-air battery. Separate zincoxide powder from current collector using mechanical means and thenplace it in KOH solution to produce K₂Zn(OH)₄ aqueous solution.

(3) Preparation of Dendritic Zinc Powder

Subject K₂Zn(OH)₄ aqueous solution to electrolysis under differentconditions and environment to produce dendritic zinc powder, which isthen electroplated at different temperatures (30, 50, and 70° C.) andspecific current density of 100˜250 mA/cm². It is found that temperaturehas significant influence on the micro-structure of electroplated zincpowder, the higher the temperature, the larger the powder particle sizeand the higher the electroplating efficiency.

FIGS. 3˜5 depict the dendritic structure of electroplated zinc powderunder current density of 100˜250 mA/cm². FIG. 6 and FIG. 7 are the XRDdiagrams of ordinary zinc powder and dendritic zinc powder,respectively.

(4) Treatment After Electroplating

Post-electrolysis treatment of dendritic zinc powder is a highlyimportant process. If the residues of KOH solution on the surface ofzinc powder are not removed completely, the zinc powder will be oxidizedinto ZnO in the drying process, rendering it useless. Thus thepost-electroplating treatment must be dealt with great prudence. Thetreatment process entails the following steps: scrap electroplated zincpowder off from the negative plate and wash it with ultra-pure water,clean with ultrasound for 30 minutes and filter, then repeat the washingprocess until the zinc powder is thoroughly cleansed off residual KOHelectrolyte. After the zinc powder is dried, seal it with zipper bag andplace it in oven to prevent the oxidation of zinc. FIG. 8 illustratesthe preparation process for porous dendritic zinc powder with largesurface area.

(5) Preparation of Zinc Gel Anode

Weigh proper amount of inhibitor In(Ac)₃ and add it in KOH solution.Agitate the solution to let the inhibitor distribute evenly. Add properproportion of dendritic zinc powder into the gel solution just prepared,and add proper amount of ZnO according to design requirement. Put theaforesaid solution in ultrasonic device for one hour. Add in properamount of poly-acrylic polymer gelling agent and agitate evenly toobtain highly viscous gel without air bubble. This completes thepreparation of zinc anode. The preparation process for zinc gel anode isdepicted in FIG. 9.

Process for Preparing the Air Electrode

Zn-air battery needs an effective air cathode electrode to work. Thisinvention focuses on developing technology for highly efficient, thin,air cathode electrode, which entails the development of better catalyst,electrode structure with longer life, and lower production costs. FIG.10 illustrates the structural diagram of an air cathode electrode. Theair cathode electrode consists of a carbon diffusion layer, nickelscreen current and an active layer pressed together, and is separatedfrom the zinc anode by a separator. The diffusion layer of air electrodeis made of hydrophobic activated carbon and nickel-screen collector,whereas the active layer is made of hydrophilic carbon powder pluscatalysts (KMnO₄ and MnO₂).

Given the oxygen in air cannot act as an electrode to accept electronand undergo reduction reaction, it needs to undergo reaction through acarbon electrode made of active carbon carrier. The active carboncarrier does not participate in electrode reaction, but provides a venuefor oxygen to undergo cathode reduction. Air cathode electrode is lessactive in acidic and neutral medium, and the electrode materials and thecatalyst are prone to corrosion in acid medium. Therefore air cathodeelectrode in alkaline electrolyte is more extensively applied at thepresent time.

The equation for electrochemical reduction of oxygen in alkalineelectrolyte is as follows:O₂+2H₂O+4e ⁻→4OH³¹(I), E ⁰=0.410 V (vs. SHE)

Air cathode electrode has smaller exchange current density (i_(o)) thatmakes the establishment of balanced potential difficult and itspolarization is more serious under load.

The air cathode electrode is primarily a carbon electrode where oxygenis dissolved and adhered on its surface to undergo electrochemicalreaction. But oxygen's solubility in alkaline solution is small. Toincrease the working current density of the Zn-air electrode and reducepolarization that will help increase the real surface area of electrodeand reduce the boundary thickness of liquid phase transfer. The porousdiffusion electrode is designed to meet these requirements. Keeping thestability of reaction zone (usually referred to as tri-phase interfacereaction) inside the porous gas diffusion electrode is an importantissue. In cohesive gas diffusion electrode, water repellent (e.g.polytetrafluoroethylene, PTFE) is used to give the electrode certainhydrophobicity and to keep the triphase interface stable. The level ofpolytetrafluoroethylene is usually at 5˜10 wt %. Too much waterrepellent will lower the conductivity of electrode, affecting thebattery performance. For Zn-air battery that uses solid electrolyte,such as PVA-GF alkaline solid polymer, some solid-state metal oxide maybe added to the electrolyte to improve the stability of interface.

Air gas diffusion electrode is an electrode with certain porosity andhigh specific surface area and able to form stable tri-phase interfacesystem. That is why its reaction mechanism is more complicated, whichusually comprises the following steps:

Diffusion of gas→diffusion→chemical absorption→electrochemicalreaction→products diffused into solution.

Generally speaking the air cathode electrode has gas on one side andelectrolyte on the other. The liquid at tri-phase interface formsmeniscus on capillaries of the electrode and adheres to the extremelythin film on electrode surface. Although O₂ gas has very low solubilityand diffusion in liquid, oxygen is still able to penetrate the film toreach the electrode at normal speed due to the thinness of the film. Toreach triphase stability inside the electrode, its capillaries may notbe fully filled with electrolyte and at the same time must allow theentry of KOH electrolyte into them.

Ordinary electrode materials and catalyst are hydrophilic. But to meetthe requirement where the capillaries on electrode surface is neithercompletely “dry”, nor completely “moist”, so as to establish a stabletri-phase interface, water repellent must be added to change the contactangle of electrode surface. Therefore the gas diffusion electrode mustcontain at least three layers, i.e. water-proof layer, collectingnetwork, and active layer.

The method for preparing the air cathode electrode of this invention ispresented as follows:

(1) Preparation of Gas Diffusion Layer

a) Take proper amount of Triton-X surfactant, PTFE-30 solution and H₂O,and mix them uniformly, and then put the mixture together with thevessel into ultrasonic device and vibrate for 10 minutes (to let PTFE,H₂O and Triton X mix uniformly);

b) Add weighed AB50 carbon powder to the mixture, agitate manually andthen put the mixture in ultrasonic device to vibrate for 30 minutes,then place the mixture in oven to dry at 120° C. (to remove H₂Ocompletely);

c) The resulting dried materials will be lumped together; grind the lumpuniformly and weigh proper amount based on the size of the airelectrode;

d) Put the nickel screen in the die fixture and coat the materialsuniformly on the screen;

e) Put the die in thermal press and sinter under constant pressure basedon parameter requirements (time, temperature and thickness). Afterwards,send the die to cooler, remove the diffusion layer after it is cooledand wait for the spraying of active layer. (See FIG. 11).

(2) Preparation of Active Layer

a) Weigh XC-72R carbon powder and add in proper amount of catalystsKMnO₄ and MnO₂;

b) Oscillate weighed PTFE-30 and H₂O ultrasonically for 5 minutes (tomix them uniformly);

c) Add the material in step a. into that in step b) and agitate with theaid of ultrasonic device;

d) Add in proper amount of methanol and iso-propanol; agitate manuallyand then with the aid of ultrasonic device for 30 min (in liquid statefor spraying purpose). Spray the resulting liquid on diffusion layeraccording to the amount required amount;

e) Subject the aforesaid the air cathode electrode to high-temperaturesintering, then let it cool under constant pressure. (See FIG. 12).

(3) Electrical Testing of Air Cathode Electrode

In the electrical testing of the air electrode, scan from open-circuitvoltage (E_(ocv)) in the direction of cathode direction to obtain I-Vpolarization curve. For testing, apply two ABS boards each to theexterior of both sides of air cathode electrode and control theirreaction area with 1 cm² to measure the current density (mA/cm²) of theair electrode under different potentials. The testing set-up ofthree-electrode system on the air electrode is depicted in FIG. 13.

Assembly of Solid Polymer Zn-Air Battery

The solid-state alkaline PVA-GF polymer electrolyte, zinc anode gel andair cathode electrodes of this invention are assembled into a polymerZn-air battery of prismatic type, which may be applied to handset, PDA,PHS and other 3C electronic products. To determine the performance ofthe assembled Zn-air polymer battery, this invention will explore theeffect of temperature, separator and discharge rate on the electricproperties of the battery. FIG. 14 is a structural diagram of polymerZn-air battery of this invention.

The electrode reactions of the polymer Zn-air battery are as follows:

Anode:Zn+4OH⁻→Zn(OH)₄ ⁻² E ^(o)=−1.25VZn(OH)₄ ⁻²→ZnO+2OH⁻+H₂O

Cathode:O₂+2H₂O+4e⁻4OH⁻ E ^(o)=+0.40V

Overall:2Zn+O₂→2ZnO E^(o)=1.65V

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is further depicted with the illustration ofembodiments.

EMBODIMENT 1 Synthesis of PVA-GF Solid-State Alkaline PolymerElectrolyte

Weigh accurately 8.0 g of polyvinyl alcohol (PVA) and 40 g of water andplace them into reactor. Measure the weight of reactor with PVA, waterand agitator in it and record it. Agitate for one hour under ambienttemperature until PVA is completely dissolved. Dissolve 12.5 g ofpotassium hydroxide (KOH) in 10 g of water and then pour it into thereactor. Raise the reactor temperature to 70° C. and control thepolymerization time to under 30 min. Measure the weight of reactor withresulting polymer inside and record it, and spread viscous polymer ofspecific weight (about 5˜10 g polymer solution) on glass fiber (GF) andplace it in temperature/humidity chamber (control the humidity at 40 RH% and temperature at 60° C.) for one hour. After that, take it out andleave it in atmosphere for 30 min to one hour. Remove the alkalinepolymer film and weigh it to calculate its chemical composition afterdrying. Preserve the polymer film in zipper bag.

Measure the thickness of PVA-GF polymer film obtained above with digitalthickness gauge and its ionic conductivity and cyclic voltammetry withAutolab FRA (bipolar stainless steel electrodes). The result ofresistance analysis is as shown in FIG. 15, and the result of Arrheniusplot is presented in FIG. 16. The figures show that the conductivity ofPVA-GF polymer electrolyte of this preferred embodiment under ambienttemperature was 0.1408 S/cm, its activation energy for reaction was 10kJ/mole, which is much lower than the activation energy of Li PEOpolymer electrolyte proposed by M. R. Armand (22˜30 kJ/mole). Table 1displays the change of conductivity of PVA-GF electrolyte underdifferent temperatures. From the cyclic voltammetry shown in FIG. 17, itis learned that in comparison with PVA-GF polymer electrolyte and PP/PEseparator, the PVA-GF polymer electrolyte in this preferred embodimentdid not undergo any oxidation and reduction reaction within workingvoltage window of −1.4˜1.4V, i.e. there was absence of faradic currentflow. Based on the test results, PVA-GF electrolyte exhibited betterelectrochemical stability than commercially available PP/PE separator(voltage stability of −1.0V˜1.0V) and cellulose separator (voltagestability of −1.2˜1.2V) with a broader range of electrochemical voltage(i.e., 2.8V window range). TABLE 1 Conductivity of PVA-GF polymerelectrolyte at different temperatures Parameter T(° C.) Impedance (ohm)Conductivity (S/cm) −20 1.1663 0.0765 −10 1.1201 0.0796 0 1.0542 0.084610 0.9294 0.0959 20 0.6335 0.1408 30 0.5616 0.1588 40 0.4829 0.1847 500.4364 0.2043 60 0.3925 0.2272 70 0.3474 0.2567 80 0.3324 0.2683

Put PVA-GF polymer film above in zipper bag and place it under constantenvironment of 25° C. and 60% RH and measure its conductivity once aweek to test the effect of time of conductivity. FIG. 18 shows that theconductivity of the polymer electrolyte did not show significant changealong with the progression of time, and was maintained around 0.1 S/cm.This result indicates that the PVA-GF polymer electrolyte has excellentstability. Table 2 illustrates the conductivity of PVA-GF electrolyte atdifferent times. TABLE 2 Conductivities of PVA-GF electrolyte No. ofdays Conductivity 7 14 21 28 35 42 49 56 σ (S/cm) 0.1413 0.1394 0.14020.1387 0.1396 0.1411 0.1408 0.1401

EMBODIMENT 2 Preparation of Dendritic Zinc Powder

Select nickel plate as negative and positive plates and dissolve 7 wt %of ZnO in 8M KOH aqueous solution. Carry out electroplating for one hourunder different temperature (30° C., 50° C., 70° C.) and at differentcurrent densities (50 mA/cm², 100 mA/cm², 200 mA/cm², and 250 mA/cm²).Post-electroplating treatment of dendritic zinc powder is a highlyimportant process; scrap electroplated zinc powder off from the negativeplate and wash it with ultra-pure D.I. water, vibrate with ultrasounddevice for 30 min and filter, then repeat the washing until the zincpowder is thoroughly cleansed off residual electrolyte. After the zincpowder is dried, seal it with zipper bag and place it in oven to preventthe oxidation of zinc.

FIG. 19 is the graph of plating potential vs. time at constant current.It is learned that the higher the current density, the greater drop ofpotential, that is, the more serious the polarization and representinggreater consumption of energy. Table 3 depicts the plating efficiency ofzinc powder under different current densities. It is found that theelectroplated zinc powder at current density of 200 mA/cm² had thehighest efficiency (84.70%) and consumed less energy. In addition, thedensities of zinc powder were less than 7.13 (g/cm³) within the range of4.8˜5.4 (g/cm³), mainly because the porous dendritic zinc powder hasvery high specific surface area. TABLE 3 Faraday efficiency andelectroplated dendritic zinc powder density at different currentdensities Parameter Theoretical Actual weight weight of zinc of zincFaraday Zn powder i(mA/cm²) powder (g) powder (g) efficiency density(g/cm³) 100 0.7308 0.3745 51.24% 4.76 166 1.2180 0.7046 57.85% 5.08 2001.8270 1.5482 84.70% 5.26 250 1.4616 1.1239 76.89% 5.42

EMBODIMENT 3 Preparation of Zinc Gel Anode

Weigh 1% In(Ac)₃ inhibitor and add in 7M KOH solution. Agitate thesolution to let the inhibitor distribute evenly. Mix 20 wt % dendriticzinc powder, 80 wt % molten zinc alloy powder into the gel justprepared. Vibrate the aforesaid solution in ultrasonic device for onehour. Add in proper amount of poly-acrylic polymer gelling agent (e.g.CMC, PVA, and capabol) and agitate evenly to obtain highly viscous gel.This completes the preparation of zinc gel anode.

EMBODIMENT 4 Preparation of Air Electrode

Take proper amount of Triton-X, PTFE-30 solution and H₂O, mix themuniformly, and then put the mixture together with the vessel intoultrasonic device and vibrate for 10 min (to let PTFE, H₂O and Triton Xmix uniformly). Add weighed AB50 carbon powder to the mixture, agitatemanually and then put the mixture in ultrasonic device to vibrate for 30min, then place the mixture in oven to dry at 120° C. (to remove H₂Ocompletely). The resulting dried materials will be lumped together;grind the lump uniformly and then coat the material uniformly on thenickel screen current collector in the die. Put the die in thermal pressand sinter under constant pressure based on parameter requirements(time, temperature and thickness). Afterwards, put the die in cooler,remove the diffusion layer after it is cooled. Weigh XC-72R carbonpowder and add in proper amount of catalyst KMnO₄ and MnO₂. WeighPTFE-30 and H₂O and then vibrate the mixture ultrasonically for 5minutes (to mix them uniformly) into PTFE aqueous solution. Add thecarbon powder and KMnO₄ and MnO₂ powders into PTFE solution. Agitatewith the aid of ultrasonic oscillator. Add in proper amount of methanoland iso-propanol; agitate manually and then with the aid of ultrasonicdevice for 30 minutes (in liquid state for spraying purpose). Spray theresulting liquid on diffusion layer according to the required. Place thecoated specimen in the oven and sinter for 20-30 min at 350° C., andthen take out the air cathode electrode, after it cools down at constantpressure.

Carry out electrical testing on air cathode electrode completed above tounderstand its performance. Start by scanning open-circuit voltage(E_(ocv)) in the direction of cathode to obtain I-V curve of electrode.For testing, apply two ABS boards each to the exterior of both sides ofair cathode electrode and control their reaction area to 1 cm² tomeasure the current density (mA/cm²) of air cathode electrode underdifferent potentials. As shown in FIGS. 20 and 21, the higher thetemperature, the higher the polarization current of air cathodeelectrode and the lower the resistance (R_(b)), meaning the better theelectrode performance. The resistance (R_(b)) of the air electrode ofthis embodiment was at between 0.6-0.7 ohm.

EMBODIMENT 5 Preparation of Polymer Zn-Air Battery and Testing ofPerformance

(1) Comparison of Zn-Air Batteries with Different Separators

Prepare 2.5 g zinc gel containing 70 wt % zinc powder as a anode and theair electrode prepared as a cathode to assemble zinc-air batteries usingPP/PE and cellulose as separator respectively. In addition, take thePVA-GF polymer electrolyte from Embodiment 1 herein to replace theaforesaid PP/PE or cellulose separator in the assembly of anotherzinc-air battery, and compare the property of different batteries. Keepthe theoretical capacity of the batteries at 1,500 mAh and discharge atthe rate of C/10 (at 150 mA) at ambient temperature. The results areshown in FIG. 22. Table 4 compares the electrical testing of zinc-airbatteries using different electrolytes. In FIG. 22 at the discharge rateof C/10, the discharge time of Zn-air battery using PP/PE separator was7.8 hours and its utilization rate was 75%; the discharge time of Zn-airbattery using cellulose separator was 8.2 hours and its utilization ratewas merely 78.85%; and the discharge time of Zn-air battery using PVA-GFpolymer electrolyte film of Embodiment 1 herein was 8.7 hours and itsutilization rate reached 83.65%.

The reason for the significant discrepancy in utilization rate was thatthe PP/PE or cellulose used in commercially available alkaline batteryhad holes in the size of 20˜30 μm, as shown in FIG. 23. When the batterydischarged, the zinc anode would expand after discharge and the zinc wasturned into zinc oxide (ZnO), which, due to expansion and squeeze of theelectrode, would enter the other electrode along the holes and bringabout short circuit. But the pin holes of PVA-GF electrolyte with size0.1˜0.2 μm. As shown in FIG. 24, small holes can block the penetrationof Zn(OH)₄ ⁻² to prevent short circuit. When the composite PVA-GF filmelectrolyte was used as electrolyte and separator, temporarycoordination bond was formed due to the dipole force generated betweenthe polymer chain and ions, and ions were conducted through theflexibility of polymer chain. As a result, the expansion of zincelectrode wouldn't lead to short circuit due to the presence of PVA-GFpolymer electrolyte film. Thus PVA-GF has higher utilization rate thanconventional separators. TABLE 4 Discharge results of Zn-air batteriesusing different separators Cycle Zn-air + Zn-air + PP/PE Zn-air + PVA-GFsolid Item 0615 cellulose polymer electrolyte Theoretical 1560 1560 1560capacity (mAh) Discharge 150 150 150 current (mA) Discharge time 7.8 8.28.7 (hrs) Actual capacity 1170 1230 1305 (mAh) Utilization rate 75 78.8583.65 (%)

(2) Performance of Zn-Air Battery under Different Discharge Rates

Prepare 2.5 g zinc gel containing 70 wt % zinc powder as a anode, andair electrode as a cathode to assemble Zn-air batteries using PVA-GFelectrolyte of Embodiment 1 herein as electrolyte and separator. Thetheoretical capacity of battery was 1500 mAh. At the discharge rate ofC/5, the battery's utilization rate reached 82.88%; at the rate of C/10,the utilization rate of zinc electrode was 89.9%; at the rate of C/20,the utilization rate of zinc electrode could reach 91.37%. FIG. 25 isthe battery discharge curve. Table 5 depicts the results at differentC-rates. The utilization rate of Zn-air battery of this invention wasover 80% no matter whether it was discharged at high or low rate, whichwill make it a competitive primary cell in the market. TABLE 5Performance of polymer Zn-air battery of this invention at differentdischarge rates Rate Item C/5 C/10 C/20 Theoretical 1560 1560 1560capacity (mAh) Discharge 300 150 75 current (mA) Discharge time 4.319.35 19.08 (hrs) Actual capacity 1293 1402.5 1431 (mAh) Utilization rate82.88 89.90 91.37 (%)

(3) Performance of Polymer Zn-Air Battery under Different Temperature

Prepare 2.5 g zinc gel containing 70 wt % zinc powder as a anode, andthe air electrode as a cathode to assemble Zn-air battery using PVA-GFelectrolyte of Embodiment 1 herein as electrolyte and separator and testits performance under different temperature environment (0° C., 20° C.,50° C.). The theoretical capacity of the battery was 1,500 mAh. FIG. 26shows the discharge curves of battery at different temperatures. Table 6depicts the utilization rate (%) of battery under differenttemperatures. At 0° C., zinc electrode utilization rate was 75%; at 20°C., zinc electrode utilization rate was 78.65%; at 50° C., zincelectrode utilization rate was 83.65%, displaying that the batteryperforms better at higher temperature. In low-temperature environment,the battery of this invention is still able to maintain utilization rateof over 70%. TABLE 6 Performance of polymer Zn-air battery of thisinvention at different temperature T (° C.) Item 0 20 50 Theoreticalcapacity (mAh) 1560 1560 1560 Discharge current (mA) 150 150 150Discharge time (hrs) 7.8 8.2 8.7 Actual capacity (mAh) 1170 1230 1305Utilization rate (%) 75% 78.85% 83.65%

(4) Analysis of Resistance/Impedance of Battery

Prepare 2.5 g zinc gel containing 70 wt % zinc powder as a anode, andthe air electrode as a cathode assemble Zn-air battery using PVA-GFelectrolyte of Embodiment 1 herein as electrolyte and separator. UseAutolab FRA system to measure the resistance/impedance of alternatingcurrent (AC) under different temperatures (0° C., 20° C., 50° C.). FIG.27 depicts the analysis results of resistance of the solid polymerZn-air battery. Table 7 depicts the resistance values of battery underdifferent temperatures. The higher the temperature in the environment,the lower AC resistance of the battery, indicating that the battery hasless resistance at high-temperature environment, and relatively, itsperformance is better than that under low temperature, due to the factthat the resistance of PVA-GF solid polymer electrolyte is higher underlow-temperature environment as shown in Table 7. TABLE 7 Resistance ofpolymer Zn-air battery of this invention at different temperature T (°C.) Resistance 0 20 50 R_(b) (ohm) 0.32 0.225 0.125

While the invention has been described with reference to a preferredembodiment thereof, it is to be understood that modifications orvariations may be easily made without departing from the spirit of thisinvention, which is defined by the appended claims.

1-8. (canceled)
 9. Process for zinc gel anode, comprising the steps of:(a) weighing proper amount of hydrogen inhibitors, adding in alkalinemetal solution and mixing to let the inhibitor distribution uniformly;and (b) adding 1˜7 wt % dendritic zinc powder into the solution preparedin the foregoing step and vibrating the mixture in ultrasonic device,then adding 0.5˜10 wt % polymer gelling agent and mixing uniformly intogel.
 10. The process according to claim 9 wherein said hydrogeninhibitor may be zinc oxide, indium acetate, magnesium oxide, calciumoxide or barium oxide.
 11. The process according to claim 9 where saidpolymer gelling agent may be CMC, PVA, starch, poly-acrylic polymergelling agent or cellulose, etc.
 12. The process according to claim 9wherein the optimum amount of said gelling agent is 1˜2 wt %. 13-14.(canceled)